Access networks are known which connect a telecommunications service provider's central office (CO) or point-of-presence (POP) to businesses and residential subscribers. Passive optical networks (PONs) are a type of access network in which a so-called optical line terminal (OLT) is provided in a central office to transmit and receive optical signals carrying data to and from remote optical network units (ONUs) provided at or near a subscriber's or customer's premises.
In conventional PONs, one or more 1:N optical splitters may be provided to distribute optical signals from the OLT to the ONUs, and optical power combiners may be provided to aggregate optical signals transmitted by the ONUs to the OLT. Earlier PONs transmitted optical signals in a downstream direction from the OLT to the ONUs at a single wavelength, such as 1490 nm, while optical signals transmitted in an upstream direction by the ONUs to the OLT were transmitted at a different wavelength, e.g., 1310 nm.
Wavelength division multiplexing (WDM) has recently been proposed as a technique for increasing the capacity of PONs. In a WDM-based PON, a plurality of optical signals, each at a corresponding one of a plurality of wavelengths, is transmitted in both the upstream and downstream directions. Since each optical signal can carry a data stream independent of the other optical signals, the information carrying capacity of WDM-based PONs may be greater than that of earlier PONs in which optical signals are time shared by each ONU.
Since a PON may include a relatively large number of ONUs, there is a need for each ONU to be relatively inexpensive and be manufacturable based on a relatively simple design. Similarly, the cost of the OLT should also be preferably reduced.